In order to complete our previous studies on the afferent activity originating from the larynx and related to respiratory events we will record from single afferent units in the recurrent laryngeal nerve and the external branch of the superior laryngeal nerve. These 2 nerves are generally thought to supply the afferent innervation to infraglottic portions of the larynx, whereas the internal branch of the superior laryngeal nerve (from which we have previously recorded) supplies supraglottic structures. Afferent information related to transmural pressure and airflow in the larynx has been found to exert a significant role in the maintenance of upper airway patency and in regulating breathing pattern. Cold air, hypercapnic mixtures and instillation of water in the larynx have been found to have important influences on bronchomotor activity and breathing pattern. We propose to study the effects of these challenges (and that of hypoxia) on the activity of laryngeal receptors activated by air flow, transmural pressure and the contraction of intrinsic laryngeal muscles. Can we identify the water receptors and carbon dioxide receptors with any of the respiratory types we have found? Stretch receptors activity depends not only on changes of airway transmural pressure, but also on smooth muscle activity. We propose to study the relationship between stretch receptor activity and smooth muscle contraction in a preparation (isolated "in vivo" extrathoracic trachea) in which transmural pressure is kept constant and smooth muscle activity is varied (asphyxia, hypoxia, hypercapnia, capsaicin). Stretch receptor discharge is recorded without impairing the motor supply to the trachealis muscle while the dog breathes spontaneously. We will explore the relationship between transpulmonary pressure and activity of airway slowly adapting stretch receptors having different locations along the tracheobronchial tree: receptors in the most distal airways may become "isolated" from transmural pressure changes occurring elsewhere along the airways. They could, in this condition, contribute an additional reflex mechanism to lung compliance homeostasis.